The present invention relates, in general, to a ring laser having a curved waveguide cavity, and more particularly to a ring laser having at least one curved segment coupled to two straight segments, with the straight segments being joined at a partially transmitting facet, and to an improved process for making such lasers.
Advances in current monolithic integration technology have allowed lasers of complicated geometry to be fabricated, including ring lasers with a variety of cavity configurations. Examples of such ring lasers are found in U.S. Pat. Nos. 4,851,368, issued Jul. 25, 1989, 4,924,476 issued May 8, 1990, and 5,132,983, issued Jul. 21, 1992, the disclosures of which are hereby incorporated herein by reference. These patents disclose traveling wave semiconductor lasers, and more particularly ring-type lasers utilizing straight legs intersecting at facets, some of the facets having total internal reflection and at least one facet permitting emission of laser light generated in the ring laser. The patents also disclose a method of forming the lasers as ridges on a substrate, and in particular chemically assisted ion beam etching process for this purpose.
Conventional pn junction lasers utilize a semiconductor material such as gallium arsenide to form a Fabry-Perot resonant cavity having parallel, semi-reflective end faces, or facets, with the other set of faces on the cavity being roughened to suppress light energy in any modes except the mode propagating between the end faces. The junction between the n-type and p-type layers of the semiconductor forms the active region of the laser so that a bias voltage, connected across the wafer by means of metallization on the upper and lower surfaces, for example, serves to stimulate transitions between energy states within the semiconductor material, causing light to propagate along the length of the device.
A ring cavity laser possesses benefits that a Fabry-Perot cavity does not provide; for example, it provides lasing action with higher spectral purity. The development of ring cavity lasers expanded the prospective applications for integrated semiconductor lasers, and added the attractiveness of greater manufacturability and reduced cost. Such ring cavity lasers have relied on total internal reflection (TIR) facets as well as partially transmitting (PT) facets to produce traveling waves within the laser which are emitted at selected locations. However, it has been found that the use of TIR facets in such devices can lead to large optical cavities, and accordingly a new technique for fabricating ring lasers that can reduce or eliminate the reliance on TIR facets is needed.
In accordance with the present invention, a reduction in the length of ring laser cavities is obtained by providing a cavity that consists of at least one curved waveguide section and at least one partially transmitting (PT) facet. The curved segment preferably joins corresponding first ends of at least two straight waveguide segments which are joined at their second ends to form the PT facet. The curved waveguide section acts as an optical waveguide to guide laser light from one straight leg segment to the other with low loss, and partially or completely eliminates the need for TIR facets in the formation of a ring laser.
In its simplest form, the ring cavity of the present invention combines a curved waveguide with two straight waveguides and a single PT facet to form a cavity in the shape of a teardrop, when viewed in top plan view. The facet serves as an emitting surface for the laser light, and the curved shape reduces the overall length of the cavity while still retaining the higher spectral purity that is a characteristic of ring cavities.
The curved ring cavity of the invention may be fabricated as a ridge laser using known process, but alternatively may be fabricated by an improved narrow-width process which uses air or another gas as the medium outside the laser cavity. The width of the cavity produced by this process is less than 1.0 micron, and preferably about 0.2 micron for single lateral mode operation.